因为数组不能复制，所以无法编写使用数组类型形参的函数。
Because we cannot copy an array, we cannot write a function that takes an array type parameter.
在泛型接口或委托中，如果类型形参仅用作方法返回类型，而不用于方法实参，则可声明为协变的。
A type can be declared contravariant in a generic interface or delegate if it is used only as a type of method arguments and not used as a method return type.
如果接口具有逆变类型形参，则允许其方法接受与接口类型形参指定的派生类型相比，派生程度更小的类型的实参。
An interface that has a contravariant type parameter allows its methods to accept arguments of less derived types than those specified by the interface type parameter.
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